Effects of unilateral electrolytic lesion of the dorsomedial nucleus of the hypothalamus on milk-ejection reflex in the rat

The dorsomedial nucleus of the hypothalamus (DMH) has been suggested to be a final relay site for the afferent pathway of milk-ejection reflex (MER). The present experiments were undertaken to examine whether unilateral lesion of the DMH abolished MER induced by unilateral suckling by pups. The unilateral DMHs of urethane-anesthetized lactating rats were electrolytically lesioned. Then 5 pups were applied to nipples ipsilateral or contralateral to the lesioned DMH and the intramammary pressure was monitored to observe the occurrence of MER. The effects of bilateral suckling by 10 pups were also examined. Eighteen of the 29 rats displayed MER during ipsi- or bilateral suckling for about 1 h. Seventeen of these 18 rats did not show MER during contralateral suckling for about 1 h. The remaining rat that showed milk ejection during contralateral suckling had a lesion outside the DMH. In 2 additional rats, extracellular action potentials of single oxytocin (OT) cells located in the supraoptic nucleus ipsilateral to the lesioned DMH were recorded during suckling ipsi- and conralateral to the lesioned DMH. OT cells showed milk-ejection burst during ipsilateral suckling but not during contralateral suckling. The results suggest that the unilateral DMH receives information of suckling stimuli applied to contralateral nipples.